This invention relates generally to receivers of angle modulated radio frequency (RF) signals. Specifically, this invention relates to distinguishing between a desired signal and an interfering signal. More specifically, the present invention relates to devices which distinguish a desired signal from an interfering signal which exhibits nearly the same frequency as the desired signal but significantly greater amplitude.
When received energy contains a strong interfering signal component compared to the amplitude of a desired signal component and the interfering signal exhibits a frequency nearly the same as the frequency of the desired signal, such as in an adjacent channel, conventional receivers may experience trouble in distinguishing the interfering signal from the desired signal. A conventional receiver may contain a crystal bandpass filter which is tuned to the frequency of the desired signal. Such a filter greatly attenuates signals at frequencies other than the frequency of the desired signal. For example, such a filter may attenuate an interfering signal located in an adjacent channel by 60 db. However, if an interfering signal in an adjacent channel exhibits an amplitude at least 60 db greater than the amplitude of the desired signal then the interfering signal dominates and operation of the receiver may be defeated.
Some prior art radios employ spectrum spreading and correlation techniques to minimize the influence of interfering signals. However, these circuits tend to be complicated and to involve both a transmitter and a receiver portion of the radio.